1. Field of the Invention
This invention relates to an information recording-reproducing apparatus for recording and/or reproducing information on/from a recording medium such as a magnetic disc or a magneto-optical disc.
2. Related Background Art
Generally, in an information recording-reproducing apparatus such as a magnetic disc apparatus or an optical disc apparatus, when an information recording or reproduction command is received from an external controller, a process of analyzing the command is carried out and thereafter, a process for conversion from a logical address into a physical address is carried out. This conversion process includes a process for converting an address substituting the defective sector of a recording medium, and a process for converting a logical address into a physical address when the recording medium is divided into a plurality of zones and sectors constituting tracks differing from one another. When a track address for reading/writing information is thus found, seek control for moving a recording-reproducing head to a desired track is effected.
To make the recording-reproducing head seek to a desired position, it is necessary to control the velocity of the head efficiently and move the head rapidly and accurately. Generally, as a velocity control system for such a recording-reproducing head, there is adopted a control system of determining a predetermined movement schedule (velocity profile), detecting the velocity of the head in the course of seek and applying feedback so as to maintain the movement schedule. FIGS. 1A and 1B of the accompanying drawings show a popular control system, and in FIG. 1A, V.sub.ref is a reference velocity representative of the movement schedule velocity, and V.sub.n is a velocity detected in the course of seek. Also, here is shown in FIGS. 1A and 1B the relation between the velocity profile and the applied current to a carriage driving rough actuator (linear motor) for moving the head. The reference velocity V.sub.ref is a velocity calculated in conformity with the remaining distance to a target, and is found from the following equation: EQU V.sub.ref =[2.multidot..alpha.(S-.lambda./2.multidot.N)].sup.1/2,(1)
where S is the movement distance to the target, .lambda. is the track pitch, .alpha. is the deceleration acceleration, and N is the zero cross count value from which the movement distance can be known. To control the velocity of the head, a command value to the linear motor is calculated from the target velocity V.sub.ref and then the current velocity V.sub.n at each predetermined cycle, whereby feedback is applied so that the velocity of the head may follow the target velocity. The command value A.sub.ct to the linear motor is calculated from the following equation, where K is the feedback gain. EQU A.sub.ct =K(V.sub.ref -V.sub.n) (2)
Thus, as shown in FIG. 1B, the linear motor is supplied with an acceleration current at first and the velocity of the head is accelerated, and when the target velocity is reached, the current of the linear motor turns to a deceleration current, whereafter the head decelerates following the target velocity. When the head arrives at the target position, the velocity becomes 0 and thus, the seek operation is terminated.
When the velocity of the recording-reproducing head is to be detected, the detecting system is used properly in the high velocity region and the low velocity region of the velocity. Specifically, first in the high velocity region, there is used a track counting system for detecting the velocity from the number N of tracks the head has crossed within a predetermined sampling interval T.sub.s. The velocity by this track counting system is detected from the following equation: EQU V.sub.n =(.lambda./2.multidot.N)/T.sub.s ( 3)
On the other hand, in the low velocity region, there is used an inter-track counting system for detecting the zero cross points of a tracking error signal and detecting the velocity from the time T.sub.d between the zero cross points. That is, the distance between the zero cross points is 1/2 of the track pitch .lambda. and therefore, if the passage time of this 1/2 pitch is known, the velocity could be detected. The velocity V.sub.n at this time can be found from the following equation: EQU V.sub.n =(.lambda./2)T.sub.d ( 4)
Such two velocity detecting systems are selected in conformity with a predetermined reference velocity, and are changed over in such a manner that when the velocity of the head is higher than the reference velocity value, the track counting system corresponding to the high velocity region is selected and when the velocity of the head becomes lower than the reference velocity value, the inter-track counting system corresponding to the low velocity region is selected.
FIG. 2 of the accompanying drawings shows the seek sequence from after the recording-reproducing head receives a command for the access to a desired position until it arrives at the desired position. When the head receives the command, address calculation such as converting the logical address into the physical address is effected, whereafter the linear motor is driven and seek control is effected. When the linear motor is driven, a delay time (rising time) T.sub.M is created by the coil and static friction or the like of the linear motor, and the seek operation is started after this delay time T.sub.M. Thus, in the prior art, when the access of the recording-reproducing head to the desired position is to be effected, a system overhead time like the time T.sub.S required for the address calculation and the delay time T.sub.M of the linear motor have been required, besides the actual seek time, and this has been a factor which increases the whole access time.